The suspect, 37-year-old Spokane resident Aaron Farinacci, was jailed on Monday on a $US1 million ($A1.4 million) bond on a count of first-degree arson, Spokane County Sheriff John Nowels said.
Farinacci is accused of starting the Old Trails Fire, one of three that erupted on Saturday to the north and west of Spokane, by using matches or a lighter to ignite a roadside patch of grass, Nowels told reporters on Monday evening.
The origins of the two other fires remained under investigation, he said.
The three blazes together are ranked as the top firefighting priority in the US as a whole.
Dozens of major fires have for weeks been scorching the drought-parched Pacific Northwest, fouling the air with smoke and soot.
The Spokane-area fires have blackened more than 4000 hectares on the northern fringe of the state's second city, home to about 230,000 residents.
Scientists say the prolonged heat and droughts driving the growth in wildfires in North America, Europe and elsewhere in recent years are largely a function of man-made global warming, mostly caused by the use of fossil fuels.
At least 700 structures have gone up in flames since Saturday, mostly in residential areas, with another 400, including homes, damaged or destroyed, said Benjamin Cossel of the incident command centre.
Some 64,000 people were under evacuation orders on Monday, up from 4000 a day earlier.
Although no fatalities had been reported, Cossel said there was a high probability that would change, as flames spread out of wooded areas into neighbourhoods and commercial areas and search and recovery expands into currently inaccessible areas.
Nowels said the arson suspect was first detained on Saturday after a witness reported seeing a nervous-looking man bending down near some grass at the spot where arson investigators later determined the Old Trails Fire began.
Roughly 1100 firefighters were battling the Spokane blazes on Monday Cossel said, a fraction of the more than 29,200 fighting wildfires nationwide, mainly in Oregon, Washington state and Idaho, according to the National Interagency Fire Center in Boise.
The agency reported more than 100 large new wildfires on Sunday in 15 states, the bulk of them in the northwest.
In Canada, flames continued to rage across British Columbia, where record drought conditions have led to what officials over the weekend called "explosive" fire behaviour.
Fire crews in Spokane sought to capitalise on cooler, calmer weather conditions expected to prevail through Tuesday, before hot, dry winds returned later in the week, Cossel said.
The Interagency Fire Center has tallied 44,722 wildfires nationwide in 2026, the highest number for this time of the year in at least a decade, with more than 2.1 million hectares consumed, the greatest January-August area burned since 2022.
Farinacci is expected to face court on Tuesday.
